1. Field of the Invention
The present invention relates to an AIS (Alarm Indication Signal) signal transmission method and a unit for informing a loss of service in a broad band ISDN and particularly in an ATM communication system in which existing communication lines are interconnected through an intermediary of ATM, and to a transmission side ATM unit and an ATM communication system.
2. Description of the Related Art
With the late diversification of information modes including voice, data and image and the increase of information amount, a broad band ISDN (Integrated Service Digital Network) is being actively developed. For the broad band ISDN, a system for connecting existing communication networks such as LAN and LAN and PBX and PBX by an ATM (Asynchronous Transfer Mode: Asynchronous Network) is being considered.
FIG. 24 shows an arrangement of a communication network in which existing communication networks are connected by the ATM. In this communication network, existing units of routers of LAN, relay stations or PBX are connected by the ATM.
ATM units 200A and 200B and an ATM switching unit lie between an existing unit 100A on the transmission side and an existing unit 100B on the receiving side. The transmission side ATM unit 200A is connected to the transmission side existing unit 100A via an interface (DS1/DS3) in DS1 (Digital Signal Level 1) or DS3 (Digital Signal Level 3). On the other hand, the receiving side ATM unit 200B is connected to the receiving side existing unit 100B via an interface (DS1/DS3).
The ATM switching unit 300 for switching ATM cells lies between the transmission side ATM unit 200A and the receiving side ATM unit 200B. Each of the ATM units 200A and 200B and the ATM switching unit 300 are connected via user interfaces (UNI).
The interface (DS1/DS3) on the transmission side transmits a DS1 or DS3 signal transmitted from the transmission side existing unit 100A to the transmission side ATM unit 200A. The transmission side ATM unit 200A converts the DS1 or DS3 signal transmitted from the interface (DS1/DS3) into ATM cells and transmits them to the receiving side via the ATM Switching unit 300.
The receiving side ATM unit 200B restores the ATM cells received via the ATM switching unit 300 into the DS1 or DS3 signal. The interface (DS1/DS3) on the receiving side transmits the DS1 or DS3 signal sent from the receiving side ATM unit 200B to the receiving side existing unit 100B.
FIG. 25 is a functional block diagram showing an internal structure of the ATM units 200A and 200B (hereinafter generally referred to as the ATM unit 200). The conventional ATM unit 200 has transmission and receiving systems. When the transmission system receives the DS1 or DS3 signal from the interface (DS1/DS3) on the transmission side, a DS1/DS3 terminating section 201 converts the DS1 or DS3 signal into a signal mode which can be processed by the ATM unit 200 and sends it to a selector 202.
The selector 202 switches an input terminal A1 and input terminal A2 and outputs data input from the DS1/DS3 terminating section 201 or data input from an AIS generating section 204 to an ATM cell generating section 203. Normally, the input path of the selector 202 is connected to the input terminal A1 to output data input from the DS1/DS3 terminating section 201 to the ATM cell generating section 203. At this time, ATM cell generating section 203 outputs the DS1 or DS3 signal by converting into ATM cells. The ATM cells are then input to the receiving side ATM unit 200B via the ATM switching unit 300 and are restored to the DS1 or DS3 signal by the ATM unit 200 to be transmitted to the receiving side existing unit 100B (See (1) in FIG. 26).
On the other hand, a LOS detecting section 205 monitors the DS1 or DS3 signal input to the DS1/DS3 terminating section 201 and detects a loss of service (LOS) when the DS1 or DS3 frame is a signal indicating "0s" successively for a certain number of bits. When the loss of service is detected, this LOS detecting section 205 outputs an instruction to the selector 202 to switch the input terminal.
Receiving the instruction, the selector 202 switches the input terminal from A1 to A2. At this time, an AIS generating section 204 outputs an AIS (Alarm Indication Signal) indicating the loss of service. The AIS signal output from the AIS generating section 204 is input to the ATM cell generating section 203 via the selector 202.
The ATM cell generating section 203 outputs the AIS signal by converting into ATM cells. This AIS signal is then transmitted to the receiving side ATM Unit 200B via the ATM switching unit 300 and to the receiving side existing unit 100B (see (3) in FIG. 26). Thereby the receiving side existing unit 100B can recognize the loss of service on the transmission side.
When the transmission side existing unit 100A transmits the AIS signal indicating the loss of service, the transmission system of the ATM unit 200 outputs the AIS signal by converting into ATM cells. The AIS signal converted into the ATM cells is then input to the receiving side ATM unit 200B via the ATM switching unit 300. Then the receiving side ATM unit 200B restores the AIS signal converted into the ATM cells to the DS1 or DS3 signal to transmit to the receiving side existing unit 100B (see (2) in FIG. 26).
Next, in the receiving system of the ATM unit 200, an ATM cell receiving section 209 receives the ATM cells transmitted from the ATM switching unit 300. The ATM cell receiving section 209 transfers the received ATM cells to a receiving buffer 208. The receiving buffer 208 temporarily stores the ATM cells input from the ATM cell receiving section 209 and appropriately outputs them.
A DS1/DS3 terminating section 207 decomposes the ATM cells output from the receiving buffer 208 to restore to the DS1 or DS3 signal. An input path of a selector 206 is normally connected with an input terminal B1 to transmit the DS1 or DS3 signal input from the DS1/DS3 terminating section 207 to the existing unit 100.
A starvation detecting section 210 monitors a number of ATM cells stored in the receiving buffer 208 and outputs to the selector 206 an instruction to switch the input paths when it detects that the number of cells is "zero" (starvation) for a specific period of time.
At this time, the AIS generating section 211 outputs an AIS signal indicating a loss of service. Then the selector 206 inputs the AIS signal output from the AIS generating section 211 via an input terminal B2 and transmits it to the existing unit 100 (see (4) in FIG. 26).
By the way, in the conventional method and system described above, because the transmission system of the ATM unit 200 outputs an AIS signal by converting into ATM cells when a loss of service occurs, the ATM cells are transmitted on the user interfaces (UNI) even if no data to be communicated exists. That is, because communication charge is made based on the number of cells transmitted on the user interfaces (UNI) in the ATM, a user is charged even for the cells of AIS signal.